Understanding the consequences of health insurance coverage is central to evaluating proposals to expand or modify health insurance coverage in the U.S. Yet there is remarkably little convincing evidence on the impact of insuring the uninsured on their medical utilization, health outcomes, health behaviors, or overall well-being, largely because enrolling in health insurance is a choice that is made jointly with other choices that determine these outcomes. A remarkable opportunity has presented itself for providing just such evidence. For a limited window in early 2008, Oregon opened a waiting list for enrollment in its previously closed public health insurance program for certain low income adults, and then randomly drew names from the list to determine who would be given the opportunity to enroll. This unique policy environment provides researchers with a rare occasion to bring the strengths of random assignment - the standard in medical trials - to address a critical social policy question. We have already begun surveying those selected and not selected in the lottery, and propose to use NIA funding to greatly extend the reach of our data collection and analysis, including over- sampling of the near-elderly (those aged 50-64) whose health risks are in many ways similar to those aged 65 and up and who will soon age onto Medicare themselves. We propose to capitalize on this extraordinary and unique opportunity to address questions in three different areas: First, how does insurance affect health care utilization? Second, what is the effect of insurance on health? Third, how do the effects of health insurance differ for different groups? We will answer these questions by drawing on three data sources: semi-annual mail surveys, an in-person data collection effort including measurement of physiological markers of health, and administrative records on hospital and emergency department use.